Metal carbide coating material

Abstract

The present invention is a metal carbide coating material comprising a carbon base material (14) that has carbon as a main component and a metal carbide coating film that covers at least part of the carbon base material (14) and that has a metal carbide as a main component, said metal carbide coating material being characterized in that, in the thickness direction of the metal carbide coating film, between a film depth of 0% and a film depth of 80%, the carbon concentration in the metal carbide coating film increases as the film depth increases. The present invention makes it possible to provide a metal carbide coating material that enables an increase in the number of repeated uses.

Description

Carbonized Metallic Coating Materials

[0001] The present invention relates to a metal carbide coated material including a carbon substrate containing carbon as a main component and a metal carbide coating film containing metal carbide as a main component, which coats at least a portion of the carbon substrate.

[0002] Carbides such as tantalum carbide, niobium carbide, zirconium carbide, hafnium carbide, and tungsten carbide have high melting points and excellent chemical stability, strength, toughness, and corrosion resistance. Therefore, coating a carbon substrate with carbide can improve the carbon substrate's heat resistance, chemical stability, strength, toughness, corrosion resistance, and other properties. Carbide-coated materials, particularly tantalum carbide-coated materials, in which a carbide film is coated on the surface of a carbon substrate, are used as components in semiconductor single crystal manufacturing equipment for Si (silicon), SiC (silicon carbide), GaN (gallium nitride), AlN (aluminum nitride), and other semiconductor single crystals. Sublimation recrystallization is a widely known method for manufacturing bulk SiC single crystals. In this method, a crucible is filled with SiC raw material, and a SiC seed crystal is placed on top of it. A guide member is also installed around the SiC seed crystal to guide the sublimation gas to the single crystal. Sublimation gas generated by heating the SiC raw material rises along the inner wall of the guide member, and a SiC single crystal grows on the SiC seed crystal. SiC single crystal substrates used in semiconductor devices and the like are manufactured by epitaxially growing a SiC single crystal on a SiC substrate made of a bulk single crystal. Known methods for epitaxially growing SiC single crystals include liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), and chemical vapor deposition (CVD). The typical method for epitaxially growing a SiC single crystal is the CVD method. In the CVD epitaxial growth method, a SiC substrate is placed on a susceptor in an apparatus and a raw material gas is supplied at a high temperature of 1500°C or higher to grow the SiC single crystal. In order to obtain higher quality crystals in such a SiC single crystal manufacturing method, Patent Document 1 discloses a method using a crucible with a graphite base whose inner surface is coated with tantalum carbide. Furthermore, Patent Document 2 discloses a method of using a guide member whose inner wall is coated with tantalum carbide.

[0003] JP 2019-99453 A JP 2019-108611 A

[0004] It is known that SiC single crystals grown using a metal carbide-coated material as a crucible or guide member have significantly improved crystal growth yields compared to crystals grown using an uncoated carbon material. From the perspective of crystal production costs, it is desirable for the metal carbide-coated material to be reused many times. Therefore, an object of the present invention is to provide a metal carbide-coated material that can be reused many times.

[0005] As a result of extensive research, the present inventors discovered that the number of times a metal carbide coated material can be repeatedly used can be increased by focusing on the carbon concentration in the metal carbide coating film, and thus completed the present invention. The gist of the present invention is as follows: [1] A metal carbide coated material comprising a carbon substrate containing carbon as a main component and a metal carbide coating film containing metal carbide as a main component and coating at least a portion of the carbon substrate, characterized in that, when the film depth in the thickness direction of the metal carbide coating film is between 0% and 80% and is expressed by the following formula, the carbon concentration in the metal carbide coating film increases with increasing film depth: [2] A metal carbide coated material comprising a carbon substrate containing carbon as a main component and a metal carbide coating film containing metal carbide as a main component, which coats at least a portion of the carbon substrate, wherein the carbon concentration in the metal carbide coating film increases with an increase in the film depth when the film depth, expressed by the following formula, is between 20% and 80% in the thickness direction of the metal carbide coating film: [3] The metal carbide coated material according to the above [1] or [2], characterized in that the metal carbide is a carbide of at least one metal element selected from the group consisting of tantalum and niobium.

[0006] According to the present invention, it is possible to provide a metal carbide coated material that can be repeatedly used a greater number of times.

[0007] It is a schematic diagram of an externally heated low-pressure CVD apparatus. It is a schematic diagram of a low-pressure heating furnace for growing semiconductor single crystals. It is a result of GDMS analysis of the tantalum carbide coated material of Example 1. It is a result of GDMS analysis of the tantalum carbide coated material of Comparative Example 1.

[0008] The metal carbide coating material of the present invention includes a carbon substrate primarily composed of carbon and a metal carbide coating film primarily composed of metal carbide that coats at least a portion of the carbon substrate. Here, "based primarily on carbon" means that carbon accounts for 50% by mass or more of the total materials constituting the carbon substrate, and "based primarily on metal carbide" means that metal carbide accounts for 50% by mass or more of the total materials constituting the metal carbide coating film. The metal carbide coating film may be a coating film of a high-melting-point carbide such as tantalum carbide, niobium carbide, zirconium carbide, hafnium carbide, or tungsten carbide, or a composite coating film comprising two or more of these high-melting-point carbides. The metal carbide in the metal carbide coating film is preferably a carbide of at least one metal element selected from the group consisting of tantalum and niobium.

[0009] The metal carbide coated material of the present invention will be described below using a tantalum carbide coated material as an example.

[0010] [Regarding the Tantalum Carbide Coated Material] A tantalum carbide coated material according to one embodiment of the present invention will be described below with reference to Fig. 1. The tantalum carbide coated material according to one embodiment of the present invention includes a carbon substrate 14 containing carbon as a main component, and a tantalum carbide coating film containing tantalum carbide as a main component that coats the carbon substrate 14. The tantalum carbide coating film may coat a portion of the carbon substrate 14, or may coat the entire carbon substrate 14.

[0011] The carbon substrate 14 may be made of a carbon material such as isotropic graphite, extruded graphite, pyrolytic graphite, or a carbon fiber reinforced carbon composite material (C / C composite). The shape and properties of the carbon substrate 14 are not particularly limited. The carbon substrate 14 may be processed into any shape depending on the application.

[0012] The tantalum carbide coated material of one embodiment of the present invention can be produced by forming a tantalum carbide coating film on the surface of the carbon substrate 14. The tantalum carbide coating film can be formed on the surface of the carbon substrate 14 by methods such as chemical vapor deposition (CVD), sintering, and carbonization. Among these, the CVD method is preferred as a method for forming a tantalum carbide coating film because it can form a uniform and dense film.

[0013] Furthermore, CVD methods include thermal CVD, photo-assisted CVD, and plasma CVD, and for example, thermal CVD can be used to form a tantalum carbide coating film. Thermal CVD has advantages such as a relatively simple apparatus configuration and no damage caused by plasma. Formation of a tantalum carbide coating film by thermal CVD can be performed, for example, using an externally heated low-pressure CVD apparatus 10 as shown in Figure 1. In the externally heated low-pressure CVD apparatus 10, a carbon substrate 14 is supported by a support means 15 in a reaction chamber 12 equipped with a heater 13, a raw material supply unit 16, an exhaust unit 17, etc.

[0014] A method for producing a tantalum carbide coated material according to one embodiment of the present invention will be described with reference to Figure 1. First, a carbon substrate 14 is placed in a reaction chamber 12 of an externally heated low-pressure CVD apparatus 10. The carbon substrate 14 is supported by a support means 15 having a supporting portion with a pointed tip.

[0015] Next, the reaction chamber 12 is heated. For example, the reaction chamber 12 is heated under conditions of an atmospheric pressure of 10 to 1000 Pa and a temperature of 800 to 2200°C.

[0016] Next, a tantalum carbide coating film is formed on the surface of the carbon substrate 14. Methane (CH ４ ) and hydrogen (H ２ ) and tantalum pentachloride (TaCl ５) and a tantalum carbide gas such as tantalum halide gas are supplied to the reaction chamber 12. The tantalum halide gas can be generated, for example, by heating and vaporizing the tantalum halide, or by reacting tantalum metal with a halogen gas. The source gas supplied from the source supply unit 56 is then subjected to a thermal CVD reaction at a high temperature of 800 to 2200°C and a reduced pressure of 1 to 1000 Pa, to form a tantalum carbide coating film on the carbon substrate 14.

[0017] [Method for Controlling the Carbon Content in a Tantalum Carbide Coating Film] The prepared tantalum carbide coating material is annealed at a heating temperature of 2000°C or higher for a heating time of 50 hours or longer. During this process, a reducing gas atmosphere is preferably maintained in the reactor. The reducing gas is preferably a compound that does not become an impurity in SiC single crystal growth, specifically, hydrogen gas and SiCx gas. Annealing using such a gas results in decarbonization from the surface of the tantalum carbide coating film. Meanwhile, prolonged annealing causes carbon atoms in the carbon substrate to diffuse into the tantalum carbide coating film. Due to these two effects, a tantalum carbide coating film is formed in which the carbon concentration has a gradient from the film surface toward the carbon substrate. As a result, the carbon concentration in the tantalum carbide coating film increases with increasing film depth when the film depth is between 0% and 80% as expressed by the following formula: Furthermore, in the thickness direction of the tantalum carbide coating film, even when the film depth represented by the above formula is between 20% and 80%, the carbon concentration in the tantalum carbide coating film increases with increasing film depth.The film depth is set to between 20% and 80% in order to prevent the carbon in the impurities attached to the surface of the tantalum carbide coating film from being measured as carbon in the tantalum carbide coating film when measuring the carbon concentration in the tantalum carbide coating film.The distance from the film surface of the tantalum carbide coating film to the carbon substrate is specifically the distance between the film surface of the tantalum carbide coating film and the interface between the tantalum carbide coating film and the carbon substrate in the thickness direction of the tantalum carbide coating film.

[0018] The tantalum carbide coating material of one embodiment of the present invention is an example of a metal carbide coating material of the present invention, and the metal carbide coating material of the present invention is not limited to the tantalum carbide coating material of one embodiment of the present invention. Furthermore, the metal carbide of the metal carbide coating film of the present invention is not limited to tantalum carbide. Examples of metal carbides of the metal carbide coating film of the present invention include tantalum carbide, niobium carbide, zirconia carbide, hafnium carbide, and tungsten carbide. These carbides can be used alone or in combination of two or more. Among these metal carbides, tantalum carbide and niobium carbide are preferred because they have the highest melting points and are excellent in chemical stability, strength, and corrosion resistance, and tantalum carbide is more preferred.

[0019] [Film Thickness] The thickness of the metal carbide coating film is not particularly limited, but if the metal carbide coating film is too thin, gas generated from the carbon substrate may pass through the metal carbide coating film and adversely affect the semiconductor single crystal. On the other hand, if the metal carbide coating film is too thick, the film formation time may be extended, and the film formation cost may increase. Taking these factors into consideration, the thickness of the metal carbide coating film is preferably 10 μm or more and 100 μm or less, and more preferably 20 μm or more and 50 μm or less. The thickness of the metal carbide coating film here is a value measured based on cross-sectional observation of the metal carbide coating film using a scanning electron microscope (SEM).

[0020] [Carbon Concentration in the Film and Number of Reusable Uses] The metal carbide coated material of the present invention can be reused for a longer period of time by increasing the carbon concentration in the metal carbide coating film as the film depth increases between 0% and 80% or between 20% and 80%. The following explanation discusses the mechanism by which the metal carbide coated material of the present invention can be reused for a longer period of time, using a tantalum carbide coated material as an example, but this discussion does not limit the present invention. Generally, tantalum carbide has high hardness and is prone to cracking. When a tantalum carbide coated material with cracks in the tantalum carbide coating film is exposed to a high-temperature corrosive environment, the carbon substrate corrodes through the cracks in the tantalum carbide coating film, causing the tantalum carbide coating film originally present in the corroded area to peel off. Peeling of the tantalum carbide coating film makes repeated use impossible. Therefore, it is preferable that the tantalum carbide coating film be free of cracks. On the other hand, even if the thermal expansion coefficient of the tantalum carbide coating film matches that of the carbon material, cracks may occur in the tantalum carbide coating film due to differences in the shape of the carbon substrate, local thermal expansion coefficients, etc. Therefore, even with a tantalum carbide coating material whose thermal expansion coefficients are perfectly matched, there is a limit to the number of times it can be repeatedly used.

[0021] In contrast, the tantalum carbide coating film described above, in which the carbon concentration increases from the film surface toward the carbon substrate, is thought to have a surface similar to ductile metallic tantalum, and to be closer to tantalum carbide as it approaches the carbon substrate. Furthermore, in the tantalum carbide coating film of the tantalum carbide coating material of one embodiment of the present invention, the metallic tantalum layer and the tantalum carbide layer are not separated by an interface, so it is thought that the occurrence of cracks near the surface is suppressed and the same corrosion resistance as a tantalum carbide coating film in which the entire film is tantalum carbide can be exhibited.

[0022] [Description of GDMS Analysis] The contents (by mass) of Ta, Nb, Hf, Zr, W, C, O, Cl, Fe, Al, Ca, and S in a metal carbide-coated material can be measured by glow discharge mass spectrometry (GDMS) under the following measurement conditions: As an apparatus used for glow discharge mass spectrometry, for example, a glow discharge mass spectrometer (manufactured by VG Elemental, product name "VG9000") can be used. (Measurement conditions) ・Discharge Gas: Ar (7N) ・Insulator: Ceramic ・Secondary Electrode: In Orifice ・Cell: FlatCell Assembly・Nomakization: 1kV, 1.6mA ・Ion Current: Ta ~1.2×E-11 A ・Detectors: Faraday cup: 160msec ・Daly-multiplier: 500msec

[0023] Note that a film depth of 0% refers to the first sputtering position in GDMS analysis, and a film depth of 100% refers to the point in GDMS analysis where the mass ratio of the content of the main component metal element in the metal carbide coating to the content of carbon in the metal carbide coating is 1. Furthermore, an increase in carbon concentration refers to a positive slope of an approximate straight line obtained by least squares fitting the sputtering positions (number of sputterings) and elemental analysis values ​​up to a depth of 80% in an analysis having at least 10 sputtering points up to a depth of 100%.

[0024] The gradient of the carbon concentration from 0% to 80% film depth, determined by the least squares method, is preferably 100 to 20,000 mass ppm / μm, more preferably 500 to 15,000 mass ppm / μm, and even more preferably 700 to 9,000 mass ppm / μm. The gradient of the carbon concentration from 20% to 80% film depth, determined by the least squares method, is preferably 100 to 20,000 mass ppm / μm, more preferably 500 to 15,000 mass ppm / μm, and even more preferably 700 to 9,000 mass ppm / μm.

[0025] The present invention will be explained in more detail below by showing examples, but the present invention is not limited to these examples.

[0026] The tantalum carbide coated materials of Examples 1 to 10 and Comparative Example 1 were prepared as follows.

[0027] (Example 1) First, as shown in Fig. 2, isotropic graphite was processed into a bottomed cylindrical shape (crucible 21) and a truncated conical cylindrical shape (guide member 22), which were used as the carbon substrate 14. The arithmetic mean roughness Ra of the surface of the carbon substrate 14 was 6.0 µm, and the thermal expansion coefficient of the carbon substrate 14 was 7.0 × 10 －６ / °C. The thermal expansion coefficient of the carbon substrate 14 was measured using a thermomechanical analyzer (TMA7300) manufactured by Hitachi High-Tech Science Corporation, and the thermal expansion coefficient value in the temperature range from 200°C to 1200°C was taken as the thermal expansion coefficient of the carbon substrate 14.

[0028] Next, two sets of carbon substrates 14 were placed in the reaction chamber 12 of the externally heated reduced-pressure CVD apparatus 10 shown in FIG. 1 . The carbon substrates 14 were supported by a support means 15 having three support portions with pointed tips. The tips of the support portions contact the outer surface of the carbon substrate 14 for the truncated conical carbon substrate 14, the outer surface of the carbon substrate 14 for the bottomed cylindrical carbon substrate 14, the lower surface of the carbon substrate 14 for the disk-shaped carbon substrate 14, and the outer side surface of the carbon substrate 14 for the cylindrical carbon substrate 14. Although only a bottomed cylindrical shape (crucible) is placed as the carbon substrate 14 in the reaction chamber 12 of the externally heated reduced-pressure CVD apparatus 10 shown in FIG. 1 , in reality, two sets of a bottomed cylindrical shape (crucible) and a truncated conical cylindrical shape (guide member) were placed as the carbon substrates 14.

[0029] Next, methane (CH ４ ) gas at 0.25 SLM, argon (Ar) gas at 1.0 SLM, hydrogen (H ２ Tantalum pentachloride (TaCl) gas was vaporized by heating it to 0.125 SLM and 220°C. ５ The gas was supplied at 0.25 SLM and reacted at a pressure of 100 Pa and a temperature of 1250° C. in the reaction chamber 12 to form a tantalum carbide coating film on the entire surface of the carbon substrate 14 .

[0030] The carbon substrate 14 coated with the tantalum carbide coating film was removed from the reaction chamber 12, and a crucible and a guide member made of a tantalum carbide-coated carbon material were completed. The removed carbon substrate 14 was placed back into the reaction chamber 12, and the carbon substrate 14 was heated to 2000°C. Thereafter, hydrogen (H ２ ) gas was introduced at 0.125 SLM, and the internal pressure of the reaction chamber 12 was maintained at 5000 Pa abs while heating for 50 hours to perform an annealing treatment step, thereby producing two sets of samples (crucible and guide member).

[0031] Of the two sets of samples produced, one sample (crucible and guide member) was destroyed, and the film thickness was calculated from cross-sectional observation of the tantalum carbide coating film using a scanning electron microscope (SEM). Furthermore, the carbon concentration in the tantalum carbide coating film was evaluated using glow discharge mass spectrometry (GDMS). The results are shown in Figure 4. Separately, cracks occurring in the tantalum carbide coating film were visualized by spraying a flaw detection agent onto the sample, and the presence or absence of cracks was investigated.

[0032] For the other sample (crucible and guide member), a fabricated crucible 21 and guide member 22 were placed in a reduced-pressure heating furnace 20 as shown in Figure 2, and a SiC single crystal was grown by sublimation recrystallization. A SiC raw material 25 was placed in the crucible 21, and a SiC seed crystal 24 with a diameter of 2 inches was placed above it. Argon gas was flowed into the reduced-pressure heating furnace 20 at 10 to 30 SLM, the pressure was set to 500 to 1000 Pa, and the temperature was set to 2000 to 2500°C, and the SiC raw material 25 was sublimated, and a SiC single crystal with a thickness of 5 mm was grown on the SiC seed crystal 24.

[0033] The production of SiC single crystals was repeated multiple times to confirm the number of times the crucible 21 and guide member 22 had been repeatedly used. As a result, peeling of the tantalum carbide coating was confirmed after seven uses, making it necessary to replace the members with new ones. These conditions and results are shown in Table 1. The results of Example 1 are also shown in Figure 3. The vertical axis represents concentration (ppm by mass) and the horizontal axis represents depth (μm). The depth was calculated based on the actual measured value of the crater depth after analysis, assuming that the sputtering rate during analysis was constant. Note that, since unevenness of several μm was present on the crater bottom after analysis, it is thought that there may be an error in the depth resolution.

[0034] The definition of "in the tantalum carbide coating film" is as follows: The actual film thickness measured by cross-sectional observation using an SEM is taken as the film thickness. The measurement point where the carbon concentration increased and the Ta concentration decreased in the GDMS analysis was defined as the interface between the tantalum carbon coating film and the carbon substrate. The first measurement was defined as a film depth of 0%, and the measurement point at the interface between the tantalum carbon coating film and the carbon substrate was defined as a film depth of 100%.

[0035] For example, in Example 1, an increase in carbon concentration and a decrease in Ta concentration were observed in the measurement of the 25 μm portion from the GDMS analysis. Therefore, the measurement from the 0th measurement (defined as 0% film depth) to the 25th measurement (defined as 100% film depth) in this measurement was defined as "inside the tantalum carbide coating film."

[0036] Furthermore, the film depth from the film surface of the tantalum carbide coating film toward the carbon substrate was defined as 0% for the outermost surface and 100% for the interface between the carbon substrate and the tantalum carbide coating film. GDMS measurements are performed in the direction of increasing film depth, but actual sputtering progresses in a roughly hemispherical shape when viewed from the cross-sectional direction. Therefore, the sputtering depth when a certain element is detected will be subject to error.

[0037] This time, the carbon concentration in the tantalum carbide coating film was the subject of investigation, so the film depth range was 0 to 80% to ensure that the carbon concentration in the tantalum carbide coating film could be accurately discussed. Similarly, the initial stage of GDMS measurement can measure impurities attached to the outermost surface. Therefore, it was considered preferable to investigate the film depth range of 20 to 80%.

[0038]

[0039] (Example 2) In the annealing treatment step, the heating temperature was set to 2100° C. The rest of the procedure was the same as in Example 1. The results are shown in Table 1.

[0040] (Example 3) In the annealing treatment step, the heating temperature was set to 2200° C. The rest of the procedure was the same as in Example 1. The results are shown in Table 1.

[0041] (Example 4) In the annealing treatment step, the heating temperature was set to 2300° C. The rest of the procedure was the same as in Example 1. The results are shown in Table 1.

[0042] Example 5 In the annealing treatment step, the heating temperature was set to 2400° C. The other operations were the same as in Example 1. The results are shown in Table 1.

[0043] (Example 6) In the annealing treatment step, the heating temperature was set to 2500° C. The rest of the procedure was the same as in Example 1. The results are shown in Table 1.

[0044] Example 7 In the annealing treatment step, the heating time was set to 100 hours. Otherwise, the same operation as in Example 1 was performed. The results are shown in Table 1.

[0045] Example 8 In the annealing treatment step, the heating time was set to 500 hours. Otherwise, the same operation as in Example 1 was performed. The results are shown in Table 1.

[0046] Example 9 In the annealing treatment step, the heating time was set to 1000 hours. Otherwise, the same operation as in Example 1 was performed. The results are shown in Table 1.

[0047] (Example 10) In the annealing process, instead of hydrogen gas, SiCx gas (SiC, Si) generated when SiC powder is sublimated is used. ２ C, SiC ２ The rest of the procedure was the same as in Example 1. The results are shown in Table 1.

[0048] (Example 11) Tantalum pentachloride (TaCl ５ ) instead of niobium pentachloride (NbCl ５ The rest of the procedure was the same as in Example 1. The results are shown in Table 1.

[0049] Comparative Example 1 The annealing treatment step was not carried out. The other operations were the same as in Example 1. The results are shown in Table 1 and FIG.

[0050] Comparing the results of Examples 1 to 10 with those of Comparative Example 1, in Examples 1 to 10 in which annealing was performed, when the film depth from the film surface to the carbon substrate was 0% to 100% in the thickness direction of the tantalum carbide coating film (film thickness 25 μm), the carbon concentration tended to increase with increasing film depth in the film depth range of 0% to 80%. On the other hand, in Comparative Example 1 in which annealing was not performed, when the film depth from the film surface to the carbon substrate was 0% to 100% in the thickness direction of the tantalum carbide coating film (film thickness 15 μm), the carbon concentration did not tend to increase with increasing film depth in the film depth range of 0% to 80%. Similar results were obtained in the film depth range of 20% to 80%. Furthermore, when the number of repeated uses in SiC single crystal production was confirmed, it was found that Examples 1 to 10 had a greater number of repeated uses than Comparative Example 1. From the above results, Examples 1 to 10, which have a gradient of increase in the carbon concentration in the film, can be reused many times compared to Comparative Example 1, which has no gradient, and are therefore suitable for reducing costs.

[0051] REFERENCE SIGNS LIST 10 Externally heated low-pressure CVD apparatus 11 Top chamber 12 Reaction chamber 13 Heater 14 Carbon substrate 15 Support means 16 Raw material supply section 17 Exhaust section 20 SiC single crystal growth apparatus 21 Crucible 22 Guide member 23 Upper lid 24 SiC seed crystal 25 SiC raw material

Claims

1. A metal carbide coated material comprising a carbon substrate mainly composed of carbon and a metal carbide coating film mainly composed of metal carbide that coats at least a portion of the carbon substrate, wherein, in the thickness direction of the metal carbide coating film, when the film depth represented by the following formula is between 0% and 80%, the carbon concentration in the metal carbide coating film increases as the film depth increases.

2. A metal carbide coated material comprising a carbon substrate mainly composed of carbon and a metal carbide coating film mainly composed of metal carbide that coats at least a portion of the carbon substrate, wherein the carbon concentration in the metal carbide coating film increases with increasing film depth when the film depth, expressed by the following formula, is between 20% and 80% in the thickness direction of the metal carbide coating film.

3. The metal carbide coating material according to claim 1 or 2, characterized in that the metal carbide is a carbide of at least one metallic element selected from the group consisting of tantalum and niobium.

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